He said that he had called President Obamato congratulate him on his victory, adding: "This is a time of great challenges for America and I pray that the president will be successful in guiding our nation."

Obama defeated Romney in a series of key swing states, despite the weak economic recovery and stubbornly high unemployment that dogged his campaign.

American television networks called the election for Obama late on Tuesday, but the Romney campaign team waited more than an hour to agree on the results in Ohio.